1. Field of the Invention
The present invention relates to electrode pads for a Group III nitride compound semiconductor having p-type conduction, a Group III nitride compound semiconductor device having this type of electrode pads, and methods of forming these electrode pads. In particular, the invention relates to electrode pads for a Group III nitride compound semiconductor having p-type conduction with improved adhesive strength and luminous efficiency. Further, the invention relates to a method for forming the electrode pads and preventing the side etching of a protection film when forming a window through the protection film by etching.
2. Description of Background Information
Conventionally, gold (Au) deposited on a surface of a p-type conductive gallium nitride (GaN) layer is used as an electrode. Because the adhesive strength between the Au layer and the GaN layer is weak, an electrode layer made of Au peels easily away from the GaN layer during alloying processes performed on the GaN layer.
As a means to improve the adhesive strength, an electrode with a double layer structure is employed interposing a nickel (Ni) layer between the Au electrode layer and the GaN layer.
The electrode made of the Ni layer and the Au layer formed thereon also functions as an electrode pad for wire bonding. As shown in FIG. 5A, the semiconductor device is uniformly covered with a protection layer 20 to protect the surface of the device. As shown in FIGS. 5A and 5B, mask layers 21 are formed exposing some part of the protection layer 20 over the Au layer. The exposed portion of the protection layer 20 is removed by etching to form a window 20a. A lead wire is bounded to the electrode pad of the window 20a. 
However, because the adhesive strength between the Au layer and the protection layer 20 is weak, etching liquid penetrates between them. Thus, some part of the protection layer 20, even a part which is under the mask layers 21 are removed as shown in FIG. 5B. As a result, forming the window 20a as planned is difficult.